Search

Numerous industrial applications, particularly those in the transport industry, require the joining of dissimilar materials which offers considerable benefits in terms of low cost, design flexibility, and weight reduction for overall structures. The problems associated with conventional fusion welding processes have stimulated researchers in recent years to develop new joining methods for dissimilar materials which are particularly difficult to join. Friction stir welding (FSW) originally developed for joining difficult-to-weld Al-alloys and FSSW (a variant of FSW for spot welding) have exhibited great potential for obtaining sound joints in various dissimilar alloy systems in different configurations namely butt-, lap- and spot-welding, particularly in dissimilar Al-alloys systems with different properties, which are very difficult to weld using conventional fusion welding techniques. A major difficulty in joining dissimilar Al-alloys by FSW/FSSW lies in the discontinuity in mechanical and technological properties (such as high-temperature strength, plastic deformation capacity, viscosity, etc.) of the materials to be welded across the abutting surfaces. This discontinuity as well as inherent asymmetry in heat generation and material flow of FWS/FSSW processes causes a higher asymmetry in materials flow behavior in dissimilar welding. However, it is relatively easier to implement the FSW/FSSW process to dissimilar Al-alloys in contrast to FSW of dissimilar materials combinations with very differing properties, such as Al-alloy to Mg-alloy or Al-alloy to steel.

Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi-component alloys was characterised by synchrotron small-angle X-ray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al3ZrxV1-x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour.

The research topic of the thesis deals with the application of guided and bulk waves to monitor critical engineering structures such as composites and welds, which are commonplace in aeronautics. In the first part of the research, several Structural Health Monitoring (SHM) techniques were proposed and developed using linear and nonlinear damage sensitive features. Furthermore, a novel technique was proposed to select the optimal “damage sensitive” frequency of guided waves in a pitch-catch configuration. The selected frequency was further exploited in order to construct a nonlinear baseline-free imaging technique for visualization of a composite containing a delamination defect using a sparse array of bonded piezo ceramics. The second part of the research addresses the challenging problem of detecting kissing bonds in friction stir welded joints. Two improvements, including the optimization of immersion linear ultrasonic testing and a nonlinear pulse-inversion chirp-coded harmonic imaging technique, are proposed and verified in experiment. The C-scan fingerprints obtained by the linear technique is capable of illuminating all different types of linear root flaws in the friction stir welds. The density of nonlinear defects such as microcracks accompanying zigzag lines can be further evaluated by the nonlinear pulse-inversion technique. Experimental results were conducted on a variety of samples manufactured by The Welding Institute (TWI). status: published

The master degree final project deals with the fatigue analysis of welded joints produced by modern techniques. In this thesis, the low cycle fatigue behavior of welded joints of Aluminium Alloy AW 1050 produced by tungsten inert gas welding and friction stir welding, when subjected to cyclic loads have been investigated and compared with not welded specimens. The dependence of hysteresis loop width and accumulated plastic strains on the cycle number were found and fatigue curves were obtained. The results of the low cycle fatigue experiments were used, for comparison and deciding the best sustaining technique of welding. A detailed micro graph analysis of the fatigue fractures is also conducted to study the reason of failure and inducing of micro structural defect, leading to increase in stress concentration in the ambient zones., Magistro baigiamajame projekte nagrinėjamas moderniais metodais suvirintų sujungimų nuovargis. Šiame darbe tiriamas mažaciklis nuovargis ciklinių apkrovų veikiamų sujungimų iš aliuminio lydinio AW1050, suvirintų volframo elektrodu inertinių dujų aplinkoje ir frikcinio suvirinimo metodu, bei lyginama su nesuvirintais bandiniais. Nustatytos histerezės kilpos, sukauptos plastinės deformacijos priklausomybės nuo ciklų skaičiaus ir gautos nuovargio kreivės. Mažaciklio nuovargio eksperimentų rezultatai buvo panaudoti skirtingų suvirinimo metodų įtakos sujungimui palyginimui ir tinkamiausio metodo aliuminiui suvirinti suradimui. Pateikti ir išanalizuoti bandinių nuovarginių lūžių mikroskopiniai vaizdai, bei ištirti mikroskopiniai defektai lemiantys įtempių koncentracijos padidėjimą aplinkinėse zonose.

This paper investigates the effect of active cooling on the development of welding stresses during friction stir welding by finite element modelling. The primary aim was to see if cooling powers, corresponding to those currently obtainable using liquid CO2 cooling systems applied in practically feasible locations could bring about a significant reduction in weld residual stress. Various cooling strategies were examined. The simulations revealed that a large reduction in residual stress can be obtained, particularly at the weld line, depending on the size, power and positioning of the cooling sinks. A heat sink placed as close as possible behind the heat source had the greatest effect on reducing the build up of tensile stresses. All the approaches tended to reduce the weld centreline stress more effectively than those at the heat affected zone/thermomechanically affected zone boundary (i.e. below the edge of the tool shoulder). © 2010 Maney Publishing.

Micromechanical models for ductile failure have been developed in the 1970s and 1980s essentially to address cracking in structural applications and complement the fracture mechanics approach. Later, this approach has become attractive for physical metallurgists interested by the prediction of failure during forming operations and as a guide for the design of more ductile and/or high-toughness microstructures. Nowadays, a realistic treatment of damage evolution in complex metallic microstructures is becoming feasible when sufficiently sophisticated constitutive laws are used within the context of a multilevel modelling strategy. The current understanding and the state of the art models for the nucleation, growth and coalescence of voids are reviewed with a focus on the underlying physics. Considerations are made about the introduction of the different length scales associated with the microstructure and damage process. Two applications of the methodology are then described to illustrate the potential of the current models. The first application concerns the competition between intergranular and transgranular ductile fracture in aluminum alloys involving soft precipitate free zones along the grain boundaries. The second application concerns the modeling of ductile failure in friction stir welded joints, a problem which also involves soft and hard zones, albeit at a larger scale. (C) 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

International audience; The effects of joint line remnant (JLR), kissing bond (KB), and clearance between the sheets (Gap) on tensile and fatigue properties of 2198-T851 friction stir welds have been quantitatively evaluated with respect to a reference weld made using one single sheet. The JLR has no significant influence in the investigated conditions. KB and Gap-induced defects do not significantly influence plastic yield but may induce premature crack initiation by ductile tearing and intergranular decohesions respectively. A critical value for KB opening (280 MPa), a threshold value for fatigue crack propagation from the KB (1 MPa√m) and crack growth rates consistent with literature data have been determined.

Co-encadrement de la thèse : Thilo Morgeneyer; With the aim of reducing aircraft weight, a friction stir welded Al-Cu-Li alloy (2198-T8) is considered by aircraft manufacturers for its use in fuselage or lower wing skin applications. The purpose of this study is to assess the fatigue behaviour of friction stir welds and the influence of some weld defects on fatigue properties for a target lifetime of 105 cycles. Base material, sound welds (produced via welding in one sheet), welds bearing a joint line remnant (due to the native oxide layer present on butt surfaces before welding), a kissing bond (due to a lack of tool plunge depth), and welds with an initial gap between the two butt surfaces, were tested. For the targeted lifetime, cross-weld specimens were plastically deformed during the first half of the first cycle. During cross-weld tensile tests, macroscopic properties were not altered by weld defects, although kissing bond and GAP defects acted as fracture initiation sites. In situ SEM tensile tests showed a threshold stress, beyond which the opening of the kissing bond is observed. The study of the tensile behaviour of the weld is enriched by a 3D finite element simulation. S-N curves showed that the joint line remnant did not generate a significant decrease in weld lifetime, whereas the kissing bond and GAP defects conducted to a decrease in the fatigue strength at 105 cycles by 17% and 28%, respectively. The study of initiation mechanisms revealed that those two defects were fatigue crack initiation sites only beyond a threshold value of stress. For higher values of stress, the fatigue crack initiation stage was shown to be reduced to the defect opening during the first half of the first cycle. The final weld fracture was studied through ductile tearing tests, revealing an evolution of defect harmfulness similar to the one found in S-N curves.; Dans un but de réduction de poids des avions, un alliage Al-Cu-Li (2198-T8) assemblé par friction-malaxage est envisagé par les avionneurs pour des applications de type fuselage et intrados de l'aile. L'objectif de cette étude est de déterminer le comportement en fatigue des soudures par friction-malaxage et l'influence de certains défauts de soudage pour une durée de vie de l'ordre de 105 cycles. Pour cela le matériau de base, des soudures réalisées pleine tôle (sans défaut), des soudures contenant un résidu de plan de joint (dû à la couche d'oxydes initialement présente sur les chants des tôles avant soudage), un "kissing bond" (dû à un manque de pénétration de l'outil) et un "GAP" (correspondant à un défaut d'accolement des tôles) ont été testés. Pour la durée de vie visée, les soudures sont déformées plastiquement durant la première moitié du premier cycle. Le comportement en traction monotone des soudures n'a montré aucune influence significative des défauts de soudage sur les propriétés des soudures, bien que le kissing bond et le GAP soient le site d'amorçage de la rupture. Des essais de traction in situ au MEB ont permis de déterminer une contrainte seuil au-delà de laquelle on observe l'ouverture du kissing bond. L'étude du comportement en traction des soudures a été complétée par une simulation 3D par éléments finis. Les courbes de Wöhler ont montré que le résidu de plan de joint n'engendrait pas de baisse significative de la durée de vie des soudures, alors que le kissing bond et le GAP conduisaient à une réduction de l'ordre de 17% et 28% respectivement sur la contrainte à 105 cycles. L'étude des mécanismes d'amorçage a révélé que ces deux défauts étaient site d'amorçage de fissures de fatigue uniquement au-delà d'une contrainte seuil. Dans ces cas, la phase d'amorçage était réduite à l'ouverture du défaut durant la première moitié du premier cycle. La rupture finale des soudures a été étudiée au travers d'essais de déchirure ductile montrant une évolution de la nocivité des défauts similaire à celle révélée par les courbes de Wöhler.

Co-encadrement de la thèse : Thilo Morgeneyer; With the aim of reducing aircraft weight, a friction stir welded Al-Cu-Li alloy (2198-T8) is considered by aircraft manufacturers for its use in fuselage or lower wing skin applications. The purpose of this study is to assess the fatigue behaviour of friction stir welds and the influence of some weld defects on fatigue properties for a target lifetime of 105 cycles. Base material, sound welds (produced via welding in one sheet), welds bearing a joint line remnant (due to the native oxide layer present on butt surfaces before welding), a kissing bond (due to a lack of tool plunge depth), and welds with an initial gap between the two butt surfaces, were tested. For the targeted lifetime, cross-weld specimens were plastically deformed during the first half of the first cycle. During cross-weld tensile tests, macroscopic properties were not altered by weld defects, although kissing bond and GAP defects acted as fracture initiation sites. In situ SEM tensile tests showed a threshold stress, beyond which the opening of the kissing bond is observed. The study of the tensile behaviour of the weld is enriched by a 3D finite element simulation. S-N curves showed that the joint line remnant did not generate a significant decrease in weld lifetime, whereas the kissing bond and GAP defects conducted to a decrease in the fatigue strength at 105 cycles by 17% and 28%, respectively. The study of initiation mechanisms revealed that those two defects were fatigue crack initiation sites only beyond a threshold value of stress. For higher values of stress, the fatigue crack initiation stage was shown to be reduced to the defect opening during the first half of the first cycle. The final weld fracture was studied through ductile tearing tests, revealing an evolution of defect harmfulness similar to the one found in S-N curves.; Dans un but de réduction de poids des avions, un alliage Al-Cu-Li (2198-T8) assemblé par friction-malaxage est envisagé par les avionneurs pour des applications de type fuselage et intrados de l'aile. L'objectif de cette étude est de déterminer le comportement en fatigue des soudures par friction-malaxage et l'influence de certains défauts de soudage pour une durée de vie de l'ordre de 105 cycles. Pour cela le matériau de base, des soudures réalisées pleine tôle (sans défaut), des soudures contenant un résidu de plan de joint (dû à la couche d'oxydes initialement présente sur les chants des tôles avant soudage), un "kissing bond" (dû à un manque de pénétration de l'outil) et un "GAP" (correspondant à un défaut d'accolement des tôles) ont été testés. Pour la durée de vie visée, les soudures sont déformées plastiquement durant la première moitié du premier cycle. Le comportement en traction monotone des soudures n'a montré aucune influence significative des défauts de soudage sur les propriétés des soudures, bien que le kissing bond et le GAP soient le site d'amorçage de la rupture. Des essais de traction in situ au MEB ont permis de déterminer une contrainte seuil au-delà de laquelle on observe l'ouverture du kissing bond. L'étude du comportement en traction des soudures a été complétée par une simulation 3D par éléments finis. Les courbes de Wöhler ont montré que le résidu de plan de joint n'engendrait pas de baisse significative de la durée de vie des soudures, alors que le kissing bond et le GAP conduisaient à une réduction de l'ordre de 17% et 28% respectivement sur la contrainte à 105 cycles. L'étude des mécanismes d'amorçage a révélé que ces deux défauts étaient site d'amorçage de fissures de fatigue uniquement au-delà d'une contrainte seuil. Dans ces cas, la phase d'amorçage était réduite à l'ouverture du défaut durant la première moitié du premier cycle. La rupture finale des soudures a été étudiée au travers d'essais de déchirure ductile montrant une évolution de la nocivité des défauts similaire à celle révélée par les courbes de Wöhler.

Aero 2009 Conference, 13th CASI Aeronautics Conference and 56th Annual General Meeting, May 5-7, 2009, Kanata, Ontario, available, unlimited, public